METEOROLOGY. 431 



(3) "Fog-rain" (drizzle) : Slight rain formed in low layers by cooling of air in contact with 



relatively cold sea oi land surfaces. 



(4) Orographical rain, formed in air-currents ascending mountains. 



This scheme has resulted from the daily forecastmg by which the 

 forecasters are obliged to give a plausible explanation for all rain 

 occurring, in order to have a working hypothesis on a scientific basis, 

 for later forecasts. The scheme accordingly contains all sorts of rain 

 occurring in the practice of Norwegian forecasters, i. e., in the meteor- 

 ology of northwestern Europe. The general results won by investiga- 

 tions of the weather in that region may probably be generalized for all 

 other parts of the temperate and polar zones. 



A subsequent paper by the same two meteorologists will treat 

 what is now generally called the ''polar front" (see previous report, 

 Year Book, No. 19, 1920), which connects a succession of cyclones. 



The second of the papers quoted above deals from a theoretical 

 point of view with the questioa of this polar front and the disturban?3es 

 propagating along it. Treating the entire atmosphere as a circular 

 vortex round the earth's axis, we can first determine the equilibrium 

 conditions of a surface of discontinuity as one which cuts the earth's 

 surface along the polar front. Then large scale disturbances in such 

 a surface may be discussed. These disturbances will have the charac- 

 ter of propagating waves, but of a peculiar kind. The shallowness 

 of the atmospheric layers compared to the wave-lengths and the 

 dominating influence of the deviating force of the earth's rotation as 

 soon as the time of oscillation becomes of the order of magnitude of 

 a day, indicate that the motion of any particle w ill consist in a revolu- 

 tion in an almost horizontal ellipse. It is shown that the waves under 

 these circumstances must propagate always from west to east and 

 that in their structure they present striking analogy to propagating 

 cyclones and anti-cyclones. This wave theory of cyclones and anti- 

 cyclones also leads to important consequences for the general atmos- 

 pheric circulation. 



